2 ANATOMY OF THE RABBIT. 
With succession has also come modification, the evidence of 
which lies not only, geologically speaking, in the relative times of 
appearance of life forms on the earth, but also in the fact that 
succession leads from primitive to specialized animals. revealing 
in a large way the same kinds of differences observable among those 
living at the present day. That the entire skeleton of a mammal 
is patterned upon the primitive skeleton of the fossil amphibia 
of the Carboniferous and Permian is evident from a comparison of 
the components part for part, but it is equally evident from com- 
parative anatomy that the viviparous condition of a higher mammal 
is founded upon an oviparous condition in lower forms even if no 
fossil evidence is forthcoming. That a mammal as an air-breathing 
vertebrate should develop gill structures in the embryonic con- 
dition, though circumstances never come about by which such 
structures are used, is in itself an important fact bearing on adult 
structure, but such a condition also shows to what extent a living 
animal carries ancestral features, whether functionally modified 
or not. 
All characters of animals have thus an evolutionary basis, the 
general nature of which is easily understood although the process 
by which they have been developed is still a matter of uncertainty. 
In comparison with one another, animals present certain resem- 
blances and differences—diagnostic features, which are used as a 
basis for classifying them into major and minor groups. In many 
cases characters of resemblance have been shown to be secondary, 
and are hence described as convergent. In some of these the 
resemblances are of a gross type, and the structures are described 
as analogous; in other cases they are exact or homoplastic. 
As a rule, however, characters of resemblance are broad marks of 
affinity, comparable to those seen on a small scale in human’ 
families, or in human races, and determined as in the latter cases 
by heredity. The chief basis of comparison of animals with one 
another is the general assumption that structures which are similar 
or identical are homogenous—of common origin On the other 
hand, their differences are chiefly marks of divergence in evolution. 
Although it is conceivable that many of the internal features of 
animals are the result of a general progressive development, more 
